1. Field of the Invention
The present invention relates to the field of electrochemistry and, more particularly, to photoelectrochemical cells having coated semiconductor electrodes which enhance the efficiency of the photoelectrochemical cell. More specifically, the present invention relates to protective coatings for semiconductor electrodes to eliminate or substantially reduce the photocorrosion of the electrodes in an aqueous electrolyte environment.
2. Description of the Prior Art
The field of photoelectrochemistry is recognized as having the potential to enable solar energy utilization to meet many of the energy needs of the future. Through the action of light, photoelectrochemical cells can be used to generate electric power and/or to synthesize fuels and desired chemicals from abundant, renewable resources such as water, nitrogen, and carbon dioxide.
Photoelectrochemical cells can be configured such that one or both electrodes are photoactive semiconductors. The electrodes are in contact with the electrolyte which may be in liquid form. A junction is formed at the semiconductor-electrolyte interface in the dark as the two phases come into electronic equilibrium such that the Fermi level of the semiconductor, E.sub.f, equals the electrochemical potential of the solution, E.sub.redox, thereby producing a barrier height which depends on the nature of the solution species and the specific semiconductor. On illumination of the semiconductor with light energy equal to or greater than that of the semiconductor band gap, electrons are promoted from the valence band to the conduction band, creating electron-hole pairs at or near the interface. The electron-hole pairs are spatially separated by the semiconductor junction barrier and are injected into the electrolyte at the respective electrodes to produce electrochemical oxidation and reduction reactions.
A major impediment to the exploitation of photoelectrochemical cells in solar energy conversion and storage is the susceptibility of small band gap semiconductor materials to photoanodic and photocathodic degradation. The photoinstability is particularly severe for small band gap semiconductors where the photogenerated holes, which reach the interface, can oxidize the semiconductor itself. In fact, all known semiconducting materials are predicted to exhibit thermodynamic instability towards anodic photodegradation. Whether or not an electrode is photostable, then, depends on the competitive rates of the thermodynamically possible reactions, namely the semiconductor decomposition reaction and the electrolyte reactions.
Examples of photoanodic decomposition reactions are illustrated in Table I.
TABLE I ______________________________________ Examples of Photoanodic Decomposition Reactions of Various Semiconductor Electrodes Semi- con- ductor Decomposition Photoanodic Process ______________________________________ Si Si + 4h.sup.+ + 2H.sub.2 O.fwdarw.SiO.sub.2 + 4H.sup.+ GaAs GaAs + 6h.sup.+ + 5H.sub.2 O.fwdarw.Ga(OH).sub.3 + HAsO.sub.2 + 6H.sup.+ GaP GaP + 6h.sup.+ + 6H.sub.2 O.fwdarw.Ga(OH).sub.3 + H.sub.3 PO.sub.3 + 6H.sup.+ CdS CdS + 2h.sup.+ .fwdarw.Cd.sup.2+ + S CdSe CdSe + 2h.sup.+ .fwdarw.Cd.sup.2+ + Se MoS.sub.2 MoS.sub.2 + 18h.sup.+ + 12H.sub.2 O.fwdarw.MoO.sub.3.sup.2- + 2SO.sub.4.sup.2- + 24H.sup.+ WO.sub.3 WO.sub.3 + 2h.sup.+ + 2H.sub.2 O.fwdarw.WO.sub.4.sup.2- + 1/2O.sub.2 + 4H.sup.+ ______________________________________
The range of approaches for suppression of the photocorrosion problem in cells for chemical production is more restricted than that for electricity generating cells. This is particularly true if the electrolyte contains an aqueous constituent. Table II illustrates some examples of typical fuel producing reactions in aqueous electrolytes.
TABLE II ______________________________________ Some endergonic fuel generation reactions starting with N.sub.2, CO.sub.2, and H.sub.2 O H.sup.o G.sup.o Reaction (kJ mol.sup.-1).sup.a (kJ mol.sup.-1).sup.a ______________________________________ H.sub.2 O(L).fwdarw.H.sub.2 (g) + 1/2 O.sub.2 (g) 286 237 CO.sub.2 (g) + H.sub.2 O(L).fwdarw. 270 286 HCOOH(L) + 1/2 O.sub.2 (g) CO.sub.2 (g) + H.sub.2 O(L).fwdarw.HCHO(g) + O.sub.2 (g) 563 522 CO.sub.2 (g) + 2H.sub.2 O(L).fwdarw. 727 703 CH.sub.3 OH(L) + 3/2O.sub.2 (g) CO.sub.2 (g) + 2H.sub.2 O(L).fwdarw.CH.sub.4 (g) + 2O.sub.2 (g) 890 818 N.sub.2 (g) + 3H.sub.2 O(L).fwdarw. 765 678 2NH.sub.3 (g) + 3/2O.sub.2 (g) CO.sub.2 (g) + H.sub.2 O(L).fwdarw. 467 480 1/6C.sub.6 H.sub.12 O.sub.6 (s) + O.sub.2 (g) ______________________________________ 1 eV = 23.06 K cal/mol = 96.485 kJ/mol 1 J. = 0.23901 cal
Water is a particularly attractive source of hydrogen for the reduction of the materials N.sub.2 and CO.sub.2 as well as for the direct generation of H.sub.2. Water can only be used, however, if the semiconductor electrodes are stable in its presence. In the example set forth in Table II, the production of energy rich materials such as H.sub.2, CH.sub.3 OH, CH.sub.2 O, CH.sub.2 O.sub.2, and NH.sub.3, is associated with O.sub.2 evolution. A major problem in photoelectrochemistry is that the oxidation of water at the photoanode of non-oxide n-type materials is thermodynamically and kinetically disfavored over the reaction of the valence- band holes with the semiconductor lattice. In fact, all known non-oxide and many oxide n-type photoanodes are susceptible to photodegradation in aqueous electrolytes.
A number of approaches have been used to control the photoinstability of the semiconductor-electrolyte interface by coating the semiconductor surface. For example, to stabilize semiconductor surfaces from photodecomposition, noncorroding layers of metals or relatively stable semiconductor films have been deposited onto the electrode surfaces. It has been reported that continuous metal films which block solvent penetration can protect n-type GaP electrodes from photocorrosion. However, if the films are too thick for the photogenerated holes to penetrate without being scattered, they assume the Fermi energy of the metal. Then, the system is equivalent to a metal electrolysis electrode in series with a metal-semiconductor Schottky barrier. In such a system, the processes at the metal-semiconductor junction control the photovoltage and not the electrolytic reactions. In general, an applied bias is required to drive the water oxidation. In other cases, the metal can form an ohmic contact that may lead to loss of the photoactivity of the semiconductor. In discontinuous metal coatings, the electrolyte contacts the semiconductor, a situation which can lead to substantial photocorrosion, particularly in aqueous systems. For example, discontinuous gold films do not seem to protect n-type GaP from photocorrosion.
Corrosion-resistant wide band gap oxide semiconductor (TiO.sub.2 and titanates mostly) coatings over narrow band gap n-type semiconductors such as GaAs, Si, CdS, GaP, and InP have been shown to impart some protection from photodecomposition. One of two problems is currently associated with the use of optically transparent wide band gap semiconducting oxide coatings: either a thick film blocks charge transmission or a thin film still allows photocorrosion.
Wrighton et al. (1978) have shown that chemical bonding of an electroactive group to an n-type semiconductor surface can reduce oxidative photocorrosion of the electrode during electrical power generation. However, the electroactive group consisted of ferrocene molecules which are not polymeric. When a polymeric material containing a catalyst has been covalently attached to the electrode surface, the polymer was not electrically conductive and the electrode was p-type. This distinction is important because with p-type electrodes, photodegradation by reductive processes is not a major problem in photoelectrochemical solar energy utilization. In the case of n-type and p-type semiconductors coated directly with thin catalytically active metal films for gaseous fuel production, the generally poor adherence of the metal to the semiconductor surface is a major impediment.
Charge conduction is generally much higher in electrically conductive polymers than in typical electroactive polymers. Accordingly, work on charge conductive polymers in the field of photoelectrochemistry has been directed towards stabilization of electrodes against photodegradation in electricity generating cells. Charge conductive polymers are known to protect certain semiconductor surfaces from photodecomposition by transmitting photogenerated holes in the semiconductor to oxidizable species in the electrolyte at a rate much higher than the thermodynamically favored rate of decomposition of the electrode. For example, R. Noufi, A. J. Frank, A. J. Nozik, J. Am. Chem. Soc., 103,1849 (1981) demonstrated that coating n-type silicon semiconductor photoelectrodes with a charge conductive polymer, such as polypyrrole, enhances stability against surface oxidation in electricity generating cells. As also reported by R. Noufi, D. Tench, L. F. Warren, J. Electrochem. Soc. Vol. 127,2310 (1980), n-type GaAs has also been coated with polypyrrole to reduce photodecomposition in electricity producing cells, although the polymer exhibited poor adhesion in aqueous electrolyte.
Other work relating to the coating of electrodes with charge conductive polymers to prevent photodegradation thereof include U.S. Pat. No. 4,461,691, the contents of which are specifically incorporated herein by reference. Frank, et al. U.S. patent application Ser. No. 06/483,040, filed Apr. 17, 1983, now U.S. Pat. No. 4,476,003, also addresses the subject matter of coating photoelectrodes with organic conducting polymers for the purpose of decreasing photodegradation thereof. The contents of this latter patent are also hereby specifically incorporated herein by reference. Both referenced patents disclose the use of catalysts in conjunction with such charge conducting polymers overcoating a semiconductor electrode.
However, despite the promising use of polypyrrole either alone or in conjunction with catalysts on selected semiconductors to suppress photodecomposition thereof, the results obtained for preventing photodecomposition of the electrode in fuel and useful chemical generating cells having aqueous electrolytes have only been partially successful. Moreover, it can be seen that the discovery of uses for various polymer coatings on photoelectrodes has been on a case by case basis because of the empirical nature of the effects on any particular semiconductor and/or the interaction with any given electrolyte environment.